1. Field of the Invention
The present invention relates to a motor driving technique, and more particularly to a control technique of a rotation speed (frequency) of a motor.
2. Description of the Related Art
A motor driving apparatus that drives a DC (direct current) motor or a spindle motor with a desired rotation speed monitors the rotation speed of the motor so as to produce a driving signal on the basis of the deviation from a desired target value. The motor driving apparatus that controls a motor as described above may employ a speed discriminator disclosed in the Japanese Patent Application (Laid-Open) No. H06-30589, for example.
Some of the speed discriminators compare a current rotation speed of the motor and a rotation speed that is a desired target value, and outputs an error signal according to the deviation as a digital value. This error signal produces a pulse-width modulation signal having a duty ratio according to the digital value of the error signal after a high-frequency component of the error signal is eliminated with the use of a low-pass digital filter. And the error signal feedback-controls the time for energizing the motor, on the basis of the pulse-width modulation signal, so as to stabilize the rotation speed at the target value.
When the digital filter is formed in an IIR (Infinite Impulse Response) type of nth order, the filter is constructed by utilizing a difference between the sampling value at the present point and the sampling value at the sampling timing before the present point. Therefore, as the sampling frequency of the error signal is high, the difference is reduced, so that the coefficient of the filter needs to be increased. Specifically, as the sampling frequency of the error signal is high, the characteristic of the digital filter is improved, thereby providing an advantage that the transient response, e.g., the starting characteristic of the motor can be improved, but causing a problem that the circuit size of the digital filter increases.